A gram is the unit of measurement used to measure mass in the metric system. It is equal to the mass of one milliliter (one thousandth of a liter) of water at the temperature where water has the highest density (4°C).

From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.

Gram, Gramme (?), n. [F. gramme, from Gr. ? that which is written, a letter, a small weight, fr. ? to write. See Graphic.]

The unit of weight in the metric system. It was intended to be exactly, and is very nearly, equivalent to the weight in a vacuum of one cubic centimeter of pure water at its maximum density. It is equal to 15.432 grains. See Grain, n., 4.

Gram degree, ∨ Gramme degree Physics, a unit of heat, being the amount of heat necessary to raise the temperature of one gram of pure water one degree centigrade. -- Gram equivalent Electrolysis, that quantity of the metal which will replace one gram of hydrogen.